CHAPTER XI.
BRACHIOPODA[128].
The observations which have been made on the developmental history of the Brachiopoda have thrown very considerable light on the systematic position of this somewhat isolated group.
Development of the Layers.
For our knowledge of the early stages in the development of the Brachiopoda we are almost entirely indebted to Kowalevsky[129] (No. [326]). His researches extend to four forms, Argiope, Terebratula, Terebratulina, and Thecidium. The early development of the first three of these takes place on one plan, and that of Thecidium on a second plan.
In Argiope, which may be taken as typical of the first group, the ova are transported into the oviducts (segmental organs) where they undergo their early development. The segmentation leads to the formation of a blastosphere, which then becomes a gastrula by invagination. The blastopore gradually narrows, and finally closes, while at the same time the archenteric cavity ([fig. 135] A) becomes divided into three lobes, a median (me) and two lateral (pv). These lobes next become completely separated, and the middle one forms the mesenteron, while the two lateral ones give rise to the body cavity, their outer walls forming the somatic mesoblast, and their inner the splanchnic ([fig. 135] B). The embryo now elongates, and becomes divided into three successive segments ([fig. 135] B), which are usually, though on insufficient grounds (vide Thecidium), regarded as equivalent to the segments of the Chætopoda. The alimentary tract is not continued into the hindermost of them.
Fig. 135. Two stages in the development of Argiope. (After Kowalevsky.)